This application claims the priority of German patent document 199 58 179.7, filed Dec. 2, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an evaporator for a polymer electrolyte membrane fuel cell system.
To evaporate media, for example water or fuel, in a polymer electrolyte membrane fuel cell system, evaporators which are heated with hot gas or another heat-transfer medium, such as heat-transfer oil, are known. Directly-heated evaporators are also known.
DE 197 20 294 discloses an evaporator for a fuel cell system in which a water/methanol mixture is evaporated at high temperatures as a result of the evaporator being heated by the heat released from a strongly exothermic reaction.
The present invention is based on the object of providing an evaporator for a polymer electrolyte membrane fuel cell system which makes it possible to utilize thermal energy in the polymer electrolyte membrane fuel cell system and which is suitable for use in a vehicle driven by a fuel cell.
According to the present invention, the evaporator is heated by the waste heat from the polymer electrolyte membrane fuel cell unit. It is preferable for the partial pressure of the medium which is to be evaporated to be reduced, so that the partial pressure is below the saturation vapor pressure at a given evaporator temperature.
As a result, the fuel cell waste heat in the cooling circuit can be utilized for the gas generation system of the fuel cell system, even though the polymer electrolyte membrane fuel cell unit is at a temperature level which in itself is not sufficient to be utilized for evaporation. As a result, the overall efficiency of the system can be increased. In addition, it is possible to relieve the load on a cooler provided in the cooling circuit.
In a further advantageous configuration, a fuel cell unit which allows an operating temperature that is higher than the boiling temperature of the medium to be evaporated is used.
The present invention is therefore particularly suitable for fuel cell systems which are used in vehicles.